The study of the relationship of genetic factors and non-genetic factors in mammary tumorigenesis in female mice continues. Studies with the different strains of mammary tumor virus (MTV) and the genetically variable inbred mouse strains will help in assessing the relative importance of these factors. Interest is also in the way the various MTV's are transmitted: principally by female or equally well by both sexes. Finally, the strains of mice that develop the interesting hormone dependent mammary gland tumors associated with pregnancy are also being investigated.